Treatment and handling of municipal wastewater sludge is most problematic. Because of colloidal dispersion, it cannot be readily filtered or the solids otherwise mechanically separated without first substantially reducing the amount of water. To accomplish this, following aerobic or anaerobic digestion the wastewater is held in ponds or lagoons for evaporating water to concentrate the sludge solids. These large lagoons often cover a square mile or more. In addition to the land surface area requirements for such lagoons compounded by the objectionable odors in the surrounding vicinity, the disposal of the resulting condensed sludge solids presents additional problems. Moreover, quantities of municipal sewage sludge are rapidly accelerating due to population increases, especially in concentrated urban areas, thus pushing present disposal methods and sites beyond capacity.
A desirable method for disposing of wet sludge solids recovered from the evaporation process is to spread the material on agricultural land in an attempt to use its valuable fertilizing constituents. Unfortunately, because of increasing high concentrations of some heavy metals in the sludge from industrialized areas, this method poses severe environmental hazards. Such metals include lead, mercury, silver, arsenic, cadmium and chromium, all considered hazardous by federal and municipal governmental authorities. These metals as well as other toxic trace metals can result in metal toxicity to plants, and are particularly dangerous for consumable green leaf vegetables. Moreover, such undesirable constituents have now been found to contaminate aquifers and other water sources as the heavy metals are leached from the sludge under natural water run-off conditions which effect is even more pronounced with acid rain. Landfill and fertilizer applications also have the additional drawbacks of human acceptance, particularly due to the odor and potential problems caused by fecal coliform bacteria, pathogens, and the like. Normally, land sprayed with such sludge must also be controlled to human access for at least about twelve months. It is to the elimination of these problems as well as to produce a highly acceptable, clean and useful fertilizer and of recovering precious and heavy metals from wastewater sewage sludge that the present invention is directed.